changing the default format to yaml

[immarvin]

fix #88

UT:

[root@briggs01 tmp]# xcat-inventory export -t site
schema_version: '1.0'
site:
  clustersite:
    SNsyncfiledir: /var/xcat/syncfiles
    auditnosyslog: '0'
    auditskipcmds: ALL
    blademaxp: '64'
    cleanupxcatpost: 'no'
    consoleondemand: 'no'
    databaseloc: /var/lib
    db2installloc: /mntdb2
    dhcpinterfaces: briggs01.pok.stglabs.ibm.com|enP34p1s0f0,,enP34p1s0f0.12,enP34p1s0f1:noboot
    dhcplease: '300'
    disjointdhcps: '0'
    dnshandler: ddns
    domain: pok.stglabs.ibm.com
    enableASMI: 'no'
    forwarders: 10.6.29.1
    fsptimeout: '0'
    hierarchicalattrs: postscripts
    installdir: /install
    ipmimaxp: '64'
    ipmiretries: '3'
    ipmitimeout: '2'
    master: 172.12.253.27
    maxssh: '8'
    nameservers: 172.12.253.27
    nodesyncfiledir: /var/xcat/node/syncfiles
    ntpservers: <xcatmaster>
    powerinterval: '0'
    ppcmaxp: '64'
    ppcretry: '3'
    ppctimeout: '0'
    runbootscripts: '1'
    secureroot: '1'
    sharedtftp: '1'
    sshbetweennodes: ALLGROUPS
    svloglocal: '0'
    syspowerinterval: '0'
    tftpdir: /tftpboot
    timezone: America/New_York
    useNmapfromMN: 'no'
    vsftp: n
    xcatconfdir: /etc/xcat
    xcatdebugmode: '0'
    xcatdport: '3001'
    xcatiport: '3002'
    xcatmaxbatchconnections: '300'
    xcatmaxconnections: '356'
    xcatsslversion: TLSv1

#Version 2.14.3 (git commit d1bf861f7bae8e322a8d5b33f0729d5c3fe0a2b7, built Mon Aug 13 06:15:45 EDT 2018)